Risk of being broken by new releases of OmniSharp

[mthalman]

The OmniSharp tests currently download the latest available release of OmniSharp to test against. This presents a risk because an update of OmniSharp can break the build at any point in time with no changes to our source code. #3527 is an example of such a break.

We should consider pinning to a specific release version of OmniSharp and have a system in place to update to newer versions as they become available.

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